<!DOCTYPE html>
<html lang="en">
<head>
<meta charset=utf-8 />
<title>Canvas Gradient</title>
<style>
body {
  background: #000;
}
canvas {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
}
</style>
</head>
<body>
<canvas height="600" width="600"></canvas>
<script>
var canvas = document.getElementsByTagName('canvas')[0],
    ctx = null,
    grad = null,
    body = document.getElementsByTagName('body')[0],
    color = 255;
    
if (canvas.getContext('2d')) {
  ctx = canvas.getContext('2d');
  ctx.clearRect(0, 0, 600, 600);
  ctx.save();
  // Create radial gradient
  grad = ctx.createRadialGradient(0,0,0,0,0,600); 
  grad.addColorStop(0, '#000');
  grad.addColorStop(1, 'rgb(' + color + ', ' + color + ', ' + color + ')');

  // assign gradients to fill
  ctx.fillStyle = grad;

  // draw 600x600 fill
  ctx.fillRect(0,0,600,600);
  ctx.save();
  
  body.onmousemove = function (event) {
    var width = window.innerWidth, 
        height = window.innerHeight, 
        x = event.clientX, 
        y = event.clientY,
        rx = 600 * x / width,
        ry = 600 * y / height;
        
    var xc = ~~(256 * x / width);
    var yc = ~~(256 * y / height);

    grad = ctx.createRadialGradient(rx, ry, 0, rx, ry, 600); 
    grad.addColorStop(0, '#000');
    grad.addColorStop(1, ['rgb(', xc, ', ', (255 - xc), ', ', yc, ')'].join(''));
    // ctx.restore();
    ctx.fillStyle = grad;
    ctx.fillRect(0,0,600,600);
    // ctx.save();
  };
}
</script>
<a href="http://github.com/remy/html5demos"><img style="position: absolute; top: 0; left: 0; border: 0;" src="http://s3.amazonaws.com/github/ribbons/forkme_left_darkblue_121621.png" alt="Fork me on GitHub" /></a>
<script>
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
<script>
try {
var pageTracker = _gat._getTracker("UA-1656750-18");
pageTracker._trackPageview();
} catch(err) {}</script>

</body>
</html>